Rupture of an intracranial aneurysm (IA) is a common cause of subarachnoid hemorrhage (SAH), a leading cause of premature death. While environmental factors are believed to play a role in the formation and rupture of IAs, familial aggregation strongly suggests a genetic component. Ten to twenty percent of patients have a positive family history for IAs and aneurysmal SAH, and first-degree relatives have up to five-fold increased risk of developing disease compared to the general population. Many regions in the human genome have been linked to familial IA risk. However, disease-causing genetic variants have not been identified. Our long-term goals are to identify genes that when mutated predispose individuals to IAs, identify molecular pathways that lead to IA disease and develop treatment strategies to block IA growth. Using genomewide linkage and exome analyses on a large family, we identified the nonsense mutation p.R450* in THSD1 encoding thrombospondin-type 1 domain-containing protein 1 that is present in all 9 family members with IA. In addition, we identified 7 potential mutations in the same gene in other IA families. We observed that knocking down Thsd1 in zebrafish or mice causes spontaneous brain hemorrhage and premature death, suggesting a role for THSD1 in maintaining cerebrovascular integrity. THSD1 encodes a protein with unknown molecular function. Notably, THSD1 is expressed in endothelial cells, which form an important cell layer in cerebral arteries. Interestingly, a hallmark in IA pathology is the loss of the intact endothelium, suggesting a role for endothelial dysfunction in IA formation. THSD1 has been shown to bind talin, a focal adhesion protein that assists in the attachment of cells to the extracellular matrix. It has also been shown that when endothelial cells detach from underlying matrix, cells undergo programmed cell death. Since Thsd1 deficiency leads to hemorrhage, we hypothesize that THSD1 is involved in maintaining the integrity of the endothelium by promoting adhesion of endothelial cells to the underlying basement membrane and, consequently, mediating cell survival. In this application, we propose to use two in vitro model systems: 1) vascular endothelial and smooth muscle cells isolated from wild type and Thsd1-null mice; and 2) induced pluripotent stem cell (iPSC)-derived vascular endothelial and smooth muscle cells from wild type subjects and actual patients with the THSD1 p.R450* mutation. Using immunohistochemistry, immunoblotting, and functional assays, we aim to determine whether THSD1 localizes in focal adhesions, and if disruption of THSD1 would result in altered focal adhesion assembly, adherence to the extracellular matrix and cell survival. Successful demonstration of an abnormal phenotype will provide insight into the mechanism of THSD1-mediated IA formation and potentially identify molecular targets for preventative therapy.